If it IS a freeze plug issue, the freeze plugs located on the rear of the motor are between the engine and transmission. You have to pull out the transmission to change them. If you would like to bring you truck to one of my locations, we will check it out for free and tell you what needs to be done.
3 potential components to check. most common on older 350's is heater hose nipple that screws into the right rear of intake manifold. 2nd is intake manifold gasket blown at coolant ports(front or rear) if it is a freeze plug(rare) then mr. trans is correct you must remove transmission to access freeze plugs.